The present invention relates to probes that are used in printed circuit board test fixtures and more particularly, to a socketless, leaktight replaceable probe for use in a test fixture.
It is known to provide testing fixtures for printed circuits boards (PCBs) and the like, for testing the integrity of the electrical connectivity of the circuit boards. A conventional embodiment of a testing apparatus is shown in FIG. 1, and includes a fixed, stationary substantially horizontal probe plate 10 and an overlying vertically spaced movable top plate 12. Top plate 12 is linked to probe plate 10 by means of a peripheral elastomeric spacer 14 which allows top plate 12 to vertically move towards probe plate 10 upon a vacuum being created inside the sealed enclosure formed by probe plate 10, spacer 14, top plate 12, and an overlying pressure plate (not shown) sealingly engaging the upper face of top plate 12. The downward movement of top plate 12 is accomplished by elastomeric spacer 14 partly collapsing under the movable top plate 12 being sucked downwards by the vacuum.
Top plate 12 holds on its upper surface a printed circuit board 18 which is securely anchored thereto by the above-mentioned pressure plate upon the vacuum being created. Circuit board 18 is spaced from top plate 12 by means of rigid spacers 20, 22 and is aligned, relative to probe plate 10, by means of a number of alignment rods 16 which are fixedly attached to probe plate 10 and which upwardly extend through and loosely engage respective vertically registering channels 23 provided in top plate 12 to engage alignment holes provided in circuit board 18.
A number of tapered channels 24 extend transversely through top plate 12, with a test probe 26 being located under and vertically registering with each channel 24. Each test probe 26 is fixedly attached to probe plate 10 in a manner described hereinafter, and vertically extends above and below probe plate 10. Top plate channels 24 further vertically register with electrical contact points 28 to be tested on printed circuit board 18 upon engagement with the probe tip of the test probe 26. Thus, upon top plate 12 moving downward, the probe tip of the test probe 26 abuts the selected contact point 28 to be tested on the printed circuit board 18. Through the instrumentality of known software, electric current is transmitted sequentially through selected probes to test the integrity of the electrical connectivity of the printed circuit board 18.
The probes 26 of known construction are removably inserted in a sleeve (socket) 30 fixedly anchored to the probe plate 10. Sleeve 30 in turn is connected to a computer-controlled circuit which allows electrical current to be selectively transmitted therethrough. Probe 26 includes a tube in which a plunger is vertically movable under the bias of a spring, between a lower and an upper limit position. The plunger, the tube and the sleeve are all electrically connected to each other, for allowing the electric current to be transmitted to the printed circuit board. The movable plunger is continuously biased upwardly, and is downwardly forced against the bias of the spring when the printed circuit board downwardly moves against the upper tips of the probes when the vacuum is created inside the sealed enclosure. The purpose of providing a probe which is distinct from its holding sleeve is that the probe has a limited life span, and will thus have to be changed after a certain number of uses because of wear.
Three important problems exist with the above-described conventional circuit board testing apparatus:
a) The first problem is that the stationary sleeves holding the probes prevent the use of more sturdy probes for any given probe spacing. In fact, the contact points of the probes on the printed circuit boards are closely adjacent to one another, and thus the probes need to be positioned in a closely adjacent fashion. This is becoming more and more important as the miniaturization of the printed circuit boards evolves. Thus, if the contact points of the probes on a printed circuit board are very close to one another, probes of a smaller diameter need to be used to allow the probes to be positioned closer to each other. Since the sleeves carrying the probes have a larger diameter than the probes themselves, circuit board contact points which are closer to one another require sleeves of smaller diameter, and consequently probes of even smaller diameter. Probes having a very small diameter are less sturdy and more prone to accidental breakage.
b) The second problem is that the vertical alignment of the probe tips with their respective registering circuit board contact points is in practice not always achieved. Indeed, when inserting the probes inside their respective sleeves, a certain vertical angular offset may occur. The top plate channels are tapered to promote self-alignment of the probes therein; however, the probe tips may still be slightly misaligned when they protrude beyond their respective channels in the space between the top plate and the printed circuit board. The consequence of this misalignment is that the probe tips may be allowed to contact the printed circuit board in a slightly offset fashion relative to their intended respective contact points, which may result in electric current not being transmitted to the circuit board. Thus, the testing software could falsely indicate a connection error.
c) The third problem also relates to a possible misalignment between the probe tips and their corresponding intended circuit board contact points, due to the fact that the alignment rods, which are used to position the circuit board, are fixed to the probe plate. Indeed, it is possible that a misalignment of the top plate relative to the probe plate may result in the top plate through-channels being laterally offset relative to their corresponding underlying probes, since the circuit board position is determined by the alignment rods which are integrally attached to the probe plate, while the position of the through-channels depends on the position of the top plate. If the through-channels are laterally offset relative to their corresponding probes, then certain probes may be laterally deflected by the edges of their corresponding through-channels when the top plate is lowered, which may result in the tips of these deflected probes abutting against the circuit board aside from their intended position. Again, the testing software would then detect a connection error on the printed circuit board where there is none.
Reference is here also made to U.S. Pat. No. 4,885,533 assigned to the assignee of the present application which discloses a probe which, in use, is firmly engaged in an electrically conductive socket mounted tightly in a dielectric plate of a PCB testing fixture.